1. Field of the Invention
This invention relates to hydrogen-absorbing alloys for alkaline storage batteries, which are used for negative electrodes of the alkaline storage batteries, methods of manufacturing the same, and alkaline storage batteries. More particularly, a feature of the invention is to improve a hydrogen-absorbing alloy for alkaline storage batteries using a hydrogen-absorbing alloy containing at least a rare-earth element, magnesium, nickel, and aluminum to increase capacity of the alkaline storage batteries, and to lessen the degradation of the hydrogen-absorbing alloy originating from charge-discharge cycles so that cycle life of the alkaline storage batteries can be improved.
2. Description of Related Art
Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel-metal hydride storage batteries using a hydrogen-absorbing alloy for their negative electrode have drawn considerable attention from viewpoints that they have a higher capacity than nickel-cadmium storage batteries and, being free of cadmium, they are more environmentally safe.
As the nickel-metal hydride storage batteries have been increasingly used in various portable devices, further higher performance in the nickel-metal hydride storage batteries has been demanded.
In the nickel-metal hydride storage batteries, hydrogen-absorbing alloys such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu5 crystal structure as its main phase and a Laves phase hydrogen-absorbing alloy containing Ti, Zr, V and Ni have been generally used for their negative electrodes.
However, these hydrogen-absorbing alloys generally do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to further increase the capacity of the nickel-metal hydride storage batteries.
In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloy, it has been proposed to use a hydrogen-absorbing alloy having a Ce2Ni7 type or a CeNi3 type crystal structure, other than the CaCu5 type, by adding Mg or the like to the rare earth-nickel hydrogen-absorbing alloy. (See, for example, Japanese Unexamined Patent Publication No. 11-323469.)
Nevertheless, a problem in using the above-described hydrogen-absorbing alloy for a negative electrode of an alkaline storage battery has been that repeated charge-discharge cycles cause the hydrogen-absorbing alloy to deteriorate, reducing the cycle life of the alkaline storage battery.
An object of the invention is to resolve the foregoing and other problems in alkaline storage batteries using, for their negative electrodes, a hydrogen-absorbing alloy for alkaline storage batteries in which Mg or the like is added to a rare earth-nickel hydrogen-absorbing alloy to form a crystal structure other than the CaCu5 type, and specifically, an object of the invention is to suppress degradation of the hydrogen-absorbing alloy used for the negative electrodes when repeatedly charging and discharging the alkaline storage batteries, improving cycle life of the alkaline storage batteries.